1. Field of the Invention
The present invention relates to a video camera and an electronic still camera, and more particularly, to a camera having a function to detect a relative shift between the camera body and the subject image caused due to camera shake in photographing.
2. Description of the Prior Art
Typically, a solid-state image sensing device such as a charge coupled device (CCD) is used in a camera of the above-mentioned type. In the present specification, hereinafter, a relative movement between a camera body and a subject is referred to xe2x80x9cimage shiftxe2x80x9d. The precision of image shift detection by the solid-state image sensing device depends on the luminance distribution of the subject. Conversely, the precision of image shift detection can be estimated to some degree by analyzing the luminance distribution of the subject image formed on the light receiving surface of the solid-state image sensing device. Therefore, by detecting an image shift by using only a partial area of the solid-state image sensing device which is estimated to have a sufficiently high image shift detection precision, a high detection precision is obtained with a small number of data and a small amount of calculation. What is important in this case is to correctly estimate the detection precision.
As conventional technical means for compensating for camera shake in photographing with a camera of the above-mentioned type, for example, the following means are known: a means as disclosed in Japanese Laid-open Patent Application S63-166369 in which the image plane is divided into a plurality of blocks and the image shift amount is obtained for each block to determine the vector of the entire image plane (first prior art); a means as disclosed in Japanese Published Patent Application H3-65509 in which a plurality of moving subject detecting devices are provided to detect a moving subject by calculating the image shift amount for each moving subject detecting device and adding the calculated shift amounts to average them (second prior art); a means as disclosed in U.S. Pat. No. 5,386,264 in which, in an apparatus to detect an image shift based on a video signal, the image shift detection is performed only when a high-frequency component of the video signal or a sharpness exceeds a predetermined value and is not performed when the high-frequency component or the sharpness is the predetermined value or below (third prior art); and a means as disclosed in U.S. Pat. No. 5,218,442 in which the image plane is divided into a plurality of blocks and a plurality of blocks are selected from thereamong in the order of height of the contrast so that the image shift detection is performed only with respect to the selected blocks (fourth prior art).
However, in the first prior art, since the calculation of the image shift amount is performed for all the blocks and not for only a part of the blocks, the amount of calculation inevitably increases, so that a large capacity memory is required. In the second prior art, since the calculation is performed for all the moving subject detecting devices, the same problem as that of the first prior art arises. In the third prior art, the detection area is selected only by using a high-frequency component of a video signal or a sharpness and the direction of luminance variation of the subject is not used as a factor for the image shift detection. Likewise, in the fourth prior art, the detection area is selected by using the magnitude of the contrast and the direction of luminance variation of the subject is not used.
Subsequently, a specific operation to select a part of the solid-state image sensing device as the detection area will be described in more detail. Typically, the image shift detection is made with respect to two directions: the horizontal and vertical directions of the photographic image plane. It is therefore considered to select as the detection area a part where the sharpness or the contrast of the video signal in the horizontal and vertical directions is high. In the above-mentioned fourth prior art (U.S. Pat. No. 5,218,442), contrast is detected with respect to the two directions for each block and an area is selected based on the magnitude of the contrast.
Assume now that a certain part of a solid-state image sensing device receives a subject image as shown in FIG. 1. The subject image has a sufficient sharpness or contrast in the horizontal and vertical directions. Therefore, according to the above-described selection method, the part receiving this subject image may be selected as the detection area.
In actuality, however, in this subject image, since the output of the solid-state image sensing device does not vary when the subject is moved in the direction L in the figure, it is impossible to detect any image shift. Therefore, such a subject image is inappropriate for image shift detection and the part receiving it should not be selected as the detection area.
Assume that there are two subject images of different shapes as shown in FIGS. 2A and 2B. For the same reason as that of the above case, in the subject image of FIG. 2A, although the image shift detection precision is high when the subject is moved in the direction M in the figure, the image shift detection precision is inferior when the subject is moved in the direction N. This is because the luminance variation (contrast) of the subject image is great in the direction M but small in the direction N. On the contrary, in the subject image of FIG. 2B, since the direction of luminance variation is not biased, image shift detection can be made with a high precision no matter in which direction the subject is moved. Therefore, comparing these two subject images, the subject image of FIG. 2B is more suitable for image shift detection than that of FIG. 2A.
Thus, even if the sharpness or contrast of a subject image is sufficient, some subject images are suitable but others are not suitable for image shift detection according to the configuration of the subject image, in particular, the direction of luminance variation (contrast). On the contrary, in the above-mentioned prior arts, since the detection area is selected merely based on the magnitude of the sharpness or contrast, a part receiving a subject image not suitable for image shift detection may be selected.
An object of the present invention is to provide a camera having an image shift detecting function which detects an image shift by estimating the image shift detection precision with the direction of luminance variation of a subject image formed on a light receiving surface of a solid-state image sensing device for detecting an image shift and by using only a partial area of the solid-state image sensing device which is estimated to have a sufficiently high detection precision.
To achieve the above-mentioned object, according to the present invention, a camera having an image shift detecting function is provided with an optical system which forms a subject image; an image sensing device which receives the subject image formed by the optical system to convert the subject image into an electric signal; image shift detecting means for detecting a relative shift between a camera body and a subject based on an output of the image sensing device; luminance variation direction detecting means for detecting a direction of luminance variation of the subject image based on the output of the image sensing device; selecting means for selecting a partial area of the image sensing device based on an output of the luminance variation direction detecting means; and controlling means for controlling the image shift detecting means to detect an image shift by using only an output corresponding to the partial area of the image sensing device selected by the selecting means.
According to such features, since in addition to the conventionally-used contrast, the direction of luminance variation of a subject image formed on the light receiving surface of an image sensing device is used to estimate the detection precision of a solid-state image sensing device which detects an image shift, the detection precision is more correctly estimated compared to the above-described prior arts. In addition, by detecting an image shift by using only a partial area of the solid-state image sensing device where the detection precision is sufficiently high, the image shift detection is more precisely made with a smaller amount of calculation.